Projected long-term impact of adjuvant trastuzumab emtansine (T-DM1) on metastatic breast cancer occurrence in Turkey.

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e12523 Background: Progression from early stage breast cancer (eBC) to metastatic breast cancer (mBC) constitutes a substantial disease burden in Turkey. At a population-level, investment in treatment with curative intent may contribute to long-term reductions or delays in mBC. Women diagnosed with human epidermal growth factor receptor 2 (HER2)-positive eBC who do not achieve pathologic complete response (pCR) after neoadjuvant taxane and trastuzumab-based treatment may be indicated for adjuvant trastuzumab emtansine (T-DM1) monotherapy. For this clinical population, the KATHERINE trial (NCT01772472) demonstrated significantly higher invasive disease-free survival among women who received adjuvant T-DM1 relative to trastuzumab (hazard ratio for invasive disease or death, 0.50; 95% confidence interval, 0.39 to 0.64; p<0.001). Methods: We developed epidemiology models based on data from cancer registries, observational studies, clinical trials, estimated population-level T-DM1 uptake, and extrapolations, by calendar year to predict the population-level number of women who will avoid mBC in Turkey over the next 10 years (i.e., 2022 to 2031). Weighted transitional probability averages were based on invasive disease-free and overall survival curves from the KATHERINE trial. Results: Over the next 10 years in Turkey, we projected that 22,597 women will be diagnosed with HER2-positive eBC and will not achieve pCR after neoadjuvant therapy. We projected that the total number of women who will experience mBC occurrence following adjuvant T-DM1 use from 2022-2031 will decrease annually from 0 to 249 relative to projections without introduction of T-DM1, with a cumulative total of 1,219 women not experiencing mBC occurrence, corresponding to 17% of total projected mBC occurrence over the next decade. At year 10 (2031), we projected that 29% fewer women would experience mBC occurrence in Turkey (604 with TDM-1 vs. 853 with trastuzumab). Conclusions: Population-level improvements in HER2-positive eBC to mBC disease progression are expected over the next 10 years in Turkey following adjuvant T-DM1 monotherapy. Further research will elucidate treatment-related improvements in costs to society as a result of population-level improvements in disease outcomes for women diagnosed with eBC.[Table: see text]